Which of these numbers is composite? ${11,\ 19,\ 51,\ 73,\ 89}$
Solution: A composite number is a number that has more than two factors (including 1 and itself). 11, 19, 73, and 89 each have only two factors. The factors of 51 are 1, 3, 17, and 51. Thus, 51 is the composite number.